The present invention relates to a clamp circuit for a television receiver to be employed in an AC coupled video circuit to restore the DC level of the video signal prior to its being applied to the cathode ray tube.
It has been found in the design of the video processing circuits of the television receivers that the DC level of the video signal can be affected by power supply irregularities as well as amplifier non-linearities and changes with temperature and time. The normal solution to overcome DC level change is to AC couple the video signal and then restore the DC level just before the video signal is applied to the cathode ray tube. The effect of not having the DC level of the video signal applied to the cathode ray tube representative of the DC level of the video signal as received by the receiver is that the black level of the picture is not correct and blacks appear as gray or grays appear as black depending upon the polarity of the DC error. The scheme that is employed to avoid such errors is to clamp the video signal to the desired DC voltage level. Thus, for example, the sync tip of the video signal or the back porch of the horizontal sync interval is ofen clamped to the desired DC level.
Clamp circuits have not proven to be a problem free solution to controlling the DC level of the video signal. It is desirable to provide a stiff clamp, namely one that clamps the video signal closely to the desired voltage level. Thus, normally a diode clamp with low series resistance is employed to achieve this purpose. However, it has been found that the low resistance necessary for a simple diode hard clamp, renders the clamp sensitive to noise and also video signal having excursions exceeding sync tip. In many receivers the solution to the noise problem has been to soften the clamp by adding series resistance. The result is that in the reproduced picture, tramsitted dark colors and dark grays are rendered black on bright scenes while black objects and dark colors are reproduced too bright during the reproduction of dark scenes.
It is accordingly an object of the present invention to provide a stiff clamp circuit for a television receiver that is noise immune.
Another object of the present invention is to eliminate the affect of power supply variation on the video circuitry responsive to the clamped video signal to insure the DC level of the video remains constant as clamped.
These and other objects are achieved by the utilization of a transistor clamp circuit comprising a transistor with its emitter-collector path connected to the video circuit and the base electrode to a source of DC voltage, such that the emitter-base diode of the transistor serves to clamp the video signal for example by its sync tip to the DC voltage set by the source of DC voltage. A capacitor is connected between the base and the video circuit and is selected in conjunction with the base to AC ground resistance of the transistor to progressively short circuit the emitter-base diode with increased frequency above the sync frequency. The resistance of the voltage source and additional resistance in the base circuit insure that the capacitor will not load down the video circuit. The beta of the transistor is selected to be high so that all such resistance has minimal affect upon the stiffness of the clamp.
To insure that the video signal remains clamped to the voltage set in the base circuit of the transistor clamp, means are provided to insure that the portion of the video circuit subsequent to the clamp is not subjected to power supply variations. Such means, for a non-regulated power supply, may comprise resistive coupling of the supply into the video circuit in the form of open loop feedback to offset any such variations.